Chemical-mechanical planarization ("CMP") processes create planar surfaces in the fabrication of multi-level interconnect and ultra-high density integrated circuits. In a typical CMP process, a wafer engages a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. At least one of the wafer or polishing pad moves with respect to the other to pass the surface of the wafer over the surface of the pad. Slurry solutions generally contain small, abrasive particles of silica or alumina that mechanically remove the surface of the wafer, and chemicals that chemically remove the surface of the wafer.
CMP processes must create a uniformly planar surface on a wafer at a desired endpoint so that the geometries of the component parts of a die may be accurately positioned across the full surface of the wafer. The uniformity of the planarized surface is a function of several factors, one of which is the rate at which the thickness of the wafer decreases as it is planarized (the "polishing rate"). An excessively high polishing rate, for example, is difficult to control and often decreases the uniformity of the planar surface. Thus, in order to create a sufficiently uniform surface, it is important to control the polishing rate of the CMP process.
CMP processes must also create such uniform wafer surfaces quickly to maximize the throughput of finished microelectronic devices. The throughput of CMP processes is a function of several factors including the polishing rate of the wafer and the ability to accurately stop the CMP process at a desired endpoint. A reasonably high polishing rate generally results in a greater throughput because it requires less time to planarize a wafer. Accurately stopping the CMP process at a desired endpoint is also important to maintaining a high throughput because the thickness of the dielectric layer must be within an acceptable range; if the thickness of the dielectric layer is not within an acceptable range, the wafer must be re-planarized until it reaches a desired endpoint. Such re-planarization of a wafer significantly reduces the throughput of current CMP processes. In practice, endpoints are estimated by measuring the planarizing time to planarize the first wafer in a nm to the actual desired endpoint, and then planarizing the rest of the wafers in the run for a similar period of time. Thus, it is important to control the polishing rate to provide a consistent polishing rate from one wafer to the next.
One problem with current CMP processes is that the polishing rate varies over a large number of wafers because certain structural features on the planarizing surface of the pad vary over the life of a pad. One such structural feature is the uniformity of the distribution of filler material throughout the pad. Polishing pads can be made from a mixture of a continuous phase polymer material and a filler material. The filler material, however, may agglomerate before the mixture cures, resulting in a non-uniform distribution of the filler material in the continuous phase material. Consequently, regions on the planarizing surface of a pad with excess filler material may have a high or low polishing rate, depending upon the nature of the filler material, while regions that lack filler material have a conversely low or high polishing rate. Although many efforts have been made to distribute the filler material throughout the continuous phase material homogeneously, many pads still have a non-uniform distribution of filler material on their planarizing surface. Moreover, the non-uniform areas of the filler material are not visibly distinguishable from other areas on the pad. Accordingly, it would be desirable to determine the extent of the non-uniform areas of filler material and other structural features on the planarizing surface of a pad to determine whether the pad is acceptable.
Conventional processes for determining the structural features on the planarizing surface of a polishing pad include Shore Hardness testing and Scanning Electron Microscope (SEM) testing. In Shore Hardness testing, a machine measures the hardness of the planarizing surface at a few random points across the pad. Similarly, in SEM testing, an electron microscope photographs a few random points on the pad. One problem with Shore Hardness and SEM testing is that they only analyze a small percentage of the surface area of the planarizing surface because it takes too long to test the whole surface area of the pad. Many non-uniformities of a structural feature, therefore, are not detected by Shore Hardness or SEM testing techniques. Thus, it would be desirable to develop a process for denoting non-uniformities of selected structural features on the planarizing surface on a macro-level across the whole surface of the polishing pad.